Particle filter and method for the purification of an exhaust-gas flow

ABSTRACT

Particle filters and a method for the purification of an exhaust-gas flow are described. In one example, exhaust gas is directed solely to a first particulate filter until a predetermined condition. Then exhaust gas is directed to a second particulate filter. One particulate filter may surround the other of two particulate filters. The particulate filters may provide filtered exhaust gas even when one filter becomes at least partially loaded.

RELATED APPLICATIONS

The present application claims priority to German Patent Application No.102011003019.0 filed on Jan. 24, 2011, the entire contents of which arehereby incorporated by reference for all purposes.

FIELD

The description relates to a particle filter and to a method for thepurification of an exhaust-gas flow. In particular, the descriptionrelates to the purification or filtering of exhaust gases of an internalcombustion engine of a motor vehicle.

BACKGROUND AND SUMMARY

Particles, such as for example carbonaceous soot, generated during thecombustion of fuels may be retained by a filter in order to reduce theexhaust-gas emissions. For this purpose, the exhaust-gas flow passesthrough a filter in which the particles accumulate. A periodicregeneration of the filter is necessary in order to reduce the particleloading and to ensure a controlled level of the exhaust-gascounterpressure.

In the event of the filter approaching a storage capacity, the increasedexhaust-gas counterpressure may have an adverse effect on theperformance of the engine. Furthermore, in the case of a high sootloading, the exothermic heat generated during particulate filterregeneration may degrade parts of the exhaust system.

DE 102 06 805 A1 presents a soot filter for the purification of exhaustgases, in which the soot filter has a predetermined breaking point forreducing an exhaust-gas counterpressure prevailing in the soot filter.The predetermined breaking point may be arranged in a filter body of thefilter and/or in a bypass line of the filter. The description is basedon the object of improving the purification of exhaust gases. The objectis achieved by way of the features of the claims included herein. Thedependent claims included herein define advantageous refinements of thedescription.

In one example, the inventors herein have developed an exhaust system ofa motor vehicle, comprising: a first particulate filter; a secondparticulate filter; and a exhaust gas routing system including a firstexhaust passage through the first particulate filter and a secondexhaust passage through the second particulate filter, an inlet valvebiased in a closed position and located upstream of the first and secondparticulate filters.

The possibility of increasing vehicle emissions may be reduced byproviding a secondary exhaust path around a first particulate filterduring conditions in which particulate matter stored in a firstparticulate filter approaches a storage capacity of the firstparticulate filter. Specifically, during conditions where particulatematter stored in the particulate filter is less than a threshold amount,exhaust gases may be directed substantially solely to the firstparticulate filter to save or maintain particulate matter storagecapacity in a second particulate filter. However, when particulatematter stored in the first particulate filter exceeds the thresholdlevel, a second exhaust path may be enabled, the second exhaust pathdirecting exhaust gases to the second particulate filter. In this way,engine exhaust back pressure may be reduced until the first particulatefilter is regenerated or replaced.

The present description may provide several advantages. In particular,the approach may improve engine emissions by increasing the exhaustfiltering capacity of the vehicle exhaust system. In addition, theapproach may extend the time between vehicle service intervals and orparticulate filter regeneration. Further, in some examples, the approachmay be implemented in a cost effective configuration which may notinclude an electrically controlled exhaust gas routing system.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an example, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 shows a particulate filter and exhaust gas routing systemaccording to the description during normal operation of the particulatefilter;

FIG. 3 shows the particulate filter and exhaust gas routing systemaccording to the description during a condition where particulate matterstored in the first particulate filter is greater than a threshold; and

FIG. 4 is a flowchart for operating an engine including an exhaustsystem that can direct engine exhaust into first and second particulatefilters.

The drawings serve merely for the explanation of the description, andare not intended to restrict the description. The drawings and theindividual parts are not necessarily drawn to scale. The same referencenumerals are used to denote identical or similar parts.

DETAILED DESCRIPTION

The present description is related to operating an engine that directsexhaust gas to a particulate filter. In one non-limiting example, theengine may be configured as illustrated in FIG. 1. FIGS. 2 and 3 providea detailed view of two particulate filters and an exhaust gas routingsystem that directs exhaust gas through the two particulate filters.Exhaust gases may flow substantially solely through the firstparticulate filter as is shown in FIG. 2 when particulate matter storedin the first particulate filter is less than a threshold amount. Exhaustgases may flow through the second particulate filter as is shown in FIG.3 when particulate matter stored in the first particulate filter isgreater than the threshold amount. FIG. 4 provides a method foroperating an engine and purifying engine exhaust gases via first andsecond particulate filters.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Combustion chamber 30 is showncommunicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 and exhaust valve 54. Each intake and exhaustvalve may be operated by an intake cam 51 and an exhaust cam 53. Theposition of intake cam 51 may be determined by intake cam sensor 55. Theposition of exhaust cam 53 may be determined by exhaust cam sensor 57.

Fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Fuel injector 66 delivers liquid fuel in proportion to thepulse width of signal FPW from controller 12. Fuel is delivered to fuelinjector 66 by a fuel system (not shown) including a fuel tank, fuelpump, fuel rail (not shown). Fuel pressure delivered by the fuel systemmay be adjusted by varying a position valve regulating flow to a fuelpump (not shown). In addition, a metering valve may be located in ornear the fuel rail for closed loop fuel control. Fuel injector 66 issupplied operating current from driver 68 which responds to controller12.

Intake manifold 44 is shown communicating with optional electronicthrottle 62 which adjusts a position of throttle plate 64 to control airflow from intake boost chamber 46. Compressor 162 draws air from airintake 42 to supply boost chamber 46. Exhaust gases spin turbine 164which is coupled to compressor 162.

Combustion is initiated in combustion chamber 30 when fuel automaticallyignites as piston approaches top-dead-center compression stroke. In someexamples, a universal Exhaust Gas Oxygen (UEGO) sensor (not shown) maybe coupled to exhaust manifold 48 upstream of emissions device 70. Inother examples, the UEGO sensor may be located downstream of one or moreexhaust after treatment devices. Further, in some examples, the UEGOsensor may be replaced by a NOx sensor.

Exhaust aftertreatment device 70 can include a particulate filter, inone example. In another example, multiple emission control devices suchas catalysts and particulate filters, each with multiple bricks, can beused. Emissions aftertreatment device 70 can include a particulatefilter and an oxidation catalyst in one example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, keep alive memory 110, and aconventional data bus. Controller 12 is shown receiving various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a position sensor134 coupled to an accelerator pedal 130 for sensing accelerator positionadjusted by foot 132; a pressure sensor 80 for sensing exhaust pressureupstream of turbine 164; a pressure sensor 82 for sensing exhaustpressure downstream of turbine 164; a measurement of engine manifoldpressure (MAP) from pressure sensor 122 coupled to intake manifold 44;an engine position sensor from a Hall effect sensor 118 sensingcrankshaft 40 position; a measurement of air mass entering the enginefrom sensor 120 (e.g., a hot wire air flow meter); and a measurement ofthrottle position from sensor 58. Barometric pressure may also be sensed(sensor not shown) for processing by controller 12. In a preferredaspect of the present description, engine position sensor 118 produces apredetermined number of equally spaced pulses every revolution of thecrankshaft from which engine speed (RPM) can be determined.

In some embodiments, the engine may be coupled to an electricmotor/battery system in a hybrid vehicle. The hybrid vehicle may have aparallel configuration, series configuration, or variation orcombinations thereof.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In some examples, fuel may be injected to a cylinder aplurality of times during a single cylinder cycle. In a processhereinafter referred to as ignition, the injected fuel is ignited bycompression ignition or by known ignition means such as spark plug (notshown), resulting in combustion. During the expansion stroke, theexpanding gases push piston 36 back to BDC. Crankshaft 40 convertspiston movement into a rotational torque of the rotary shaft. Finally,during the exhaust stroke, the exhaust valve 54 opens to release thecombusted air-fuel mixture to exhaust manifold 48 and the piston returnsto TDC. Note that the above is described merely as an example, and thatintake and exhaust valve opening and/or closing timings may vary, suchas to provide positive or negative valve overlap, late intake valveclosing, or various other examples. Further, in some examples atwo-stroke cycle may be used rather than a four-stroke cycle.

According to a first aspect of the description, a particle filter for anexhaust system of a motor vehicle with an exhaust-gas path comprises afilter and comprises a secondary filter, wherein in normal operation,the exhaust-gas path runs through the filter, and wherein in the eventof an overloading or approaching storage capacity of the filter, theexhaust-gas path runs through the secondary filter. In the event of anoverloading or approaching storage capacity of the filter (e.g., storinga predetermined threshold amount of particulate matter), which may alsobe referred to here as primary filter or main filter, the secondaryfilter is activated. The secondary filter then filters the exhaust-gasflow. According to the description, in the event of an overloading orapproaching a soot filter storage capacity, the exhaust-gas flowcontinues to be filtered.

Thus, emissions regulations are still adhered to and degradation in theexhaust-gas aftertreatment system may be mitigated. In the event ofoverloading or approaching storage capacity of the soot filter, theexhaust-gas flow can run substantially only through the secondaryfilter, or through the secondary and to a certain extent through the(primary) filter. Aside from the overloading or approaching storagecapacity, it is also possible in this way for the severity of effects ofdegradation states of the filter in which the exhaust-gascounterpressure is increased to be reduced. Correspondingly, the mode inwhich the one or more secondary filters are activated may also bereferred to as a secondary operating mode. In said secondary operatingmode, the soot filter can be regenerated and/or a filter exchange can beinitiated.

The secondary filter may surround the filter. This is desirablearrangement from a flow aspect, which is furthermore space-saving andcompatible with the established installation dimensions. The particlefilter may be of cylindrical form, which harmonizes well with thetubular exhaust system.

The secondary filter may have an inlet valve and an outlet valve. By wayof the valves, or a similar means such as a flap or some other variableopening, the secondary filter can be activated. That is to say, theexhaust gas routing system can be switched from the normal mode, inwhich the exhaust-gas flow runs through the filter, into the secondarymode, in which the exhaust-gas flow runs through the secondary filter.In some example, the exhaust gas routing system may only include asingle inlet valve and no outlet valve.

The inlet valve and/or the outlet valve may be preloaded by means of aspring. The switching may be realized in a simple manner by way of theone or more springs. The spring on the inlet valve may be dimensioned soas to open up the exhaust path for the exhaust gas into the secondaryfilter when a certain exhaust-gas counterpressure is reached. The springon the outlet valve may be dimensioned so as to open up the path for theexhaust gas out of the secondary filter when the pressure in thesecondary filter is higher than the pressure in the outlet region of thefilter or of the particle filter. Thus, in some examples, the exhaustgas routing system may be passively operated without being electricallycontrolled.

A pressure sensor may be arranged in the region of the secondary filterin order to detect the loading of the secondary filter. It is therebypossible to initiate a regeneration of the secondary filter, andexchange of the particle filter and/or one or more status messages forexample to the exhaust-gas aftertreatment system and/or the enginemanagement controller, in order thereby to improve the integration intothe overall system.

A sensor may be provided for detecting the position of the inlet valveand/or of the outlet valve. By way of said sensor, the operating mode ofthe particle filter can be detected and transmitted, for information, tofurther systems. A temperature sensor may be arranged in the region ofthe secondary filter. A temperature measurement in the region of thesecondary filter may be used, together with a measurement of theexhaust-gas temperature for example at the inlet of the exhaust system,to detect the operating mode. The filter and/or the secondary filter maybe designed as a wall-flow filter or as a throughflow filter. Saidestablished filter types are highly suitable for the particle filter.

According to a second aspect of the description, a method for thepurification of an exhaust-gas flow of a combustion plant, in particularof an engine of a motor vehicle, comprises the following steps:purification of the exhaust-gas flow by means of a filter, diverting theexhaust-gas flow through a secondary filter in the event of anoverloading of the filter.

The same advantages and modifications as those described above likewiseapply here. As a result of the diversion of the exhaust-gas flow, apossibility of impairment of or degradation to the engine as a result ofan increase in the exhaust-gas counterpressure may be reduced.Particulate filter overloading or particulate matter accumulation withinthe first and/or second particulate filter may be detected from anincreased exhaust-gas counterpressure. The detection may take place byway of one or more sensors. On the other hand, the overloading may alsobe detected implicitly using the means for diverting the exhaust-gasflow, for example by means of the switching or activation of the one ormore diverting devices.

In the event of an overloading or reaching a threshold soot storagecapacity of the secondary filter, the filtering efficiency thereof mayfall, and the exhaust-gas counterpressure may remain at a normal level.This has the advantage that the exhaust-gas counterpressure is notincreased again by the secondary filter. The primary particulate filtermay be regenerated and/or a filter exchange may be initiated when theexhaust-gas flow runs through the secondary filter. It is thus possiblefor the state of the (primary) filter to be restored during theoperating time of the secondary filter. The exhaust-gas flow candirected through the primary filter again when the primary filter hasbeen regenerated and/or exchanged. In this way, the primary particlefilter is returned to the normal operating mode. This may be followed bya regeneration of the secondary filter.

Thus, the system of FIG. 1 provides for an exhaust system of a motorvehicle, comprising: a first particulate filter; a second particulatefilter; and a exhaust gas routing system including a first exhaustpassage through the first particulate filter and a second exhaustpassage through the second particulate filter, an inlet valve biased ina closed position and located upstream of the first and secondparticulate filters. The exhaust system includes where exhaust gases aredirected substantially solely to the first particulate filter whenparticulates stored in the first particulate filter are less than athreshold level, and the exhaust gas routing system supplying exhaustgases to the second particulate filter when particulates stored in thefirst particulate filter are greater than the threshold level.

The exhaust system also includes where the second particulate filtersurrounds the first particulate filter. In some examples, the exhaustsystem further comprises a preload spring biasing the inlet valve in aclosed position. Pressure exerted by the exhaust gases may overcome thespring to open the inlet valve. The exhaust system further comprises anoutlet valve positioned downstream of the second particulate filter andthe first particulate filter. The exhaust system further comprises apreload spring biasing the outlet valve in a closed position.

The exhaust system further comprises a pressure sensor positionedupstream of the first particulate filter. The pressure sensor may be thebasis for determining when to open the inlet valve. Further, thepressure sensor may be the basis for determining when to close the inletvalve. The exhaust system further comprises a pressure sensor locateddownstream of the second particulate filter. The exhaust system furthercomprises an inlet valve position sensor, an outlet valve positionsensor, and a temperature sensor positioned upstream of the secondparticulate filter.

Referring now to FIG. 2, an exhaust system or exhaust-gas aftertreatmentsystem 70 for an internal combustion engine, for example of a motorvehicle, is shown. The exhaust-gas flow 203 generated by the internalcombustion engine flows through an exhaust line 202 in the direction ofan outlet or exhaust tailpipe. Arranged in the exhaust line 202 is aparticle filter 204, which may also be a constituent part of the exhaustline 202 or of the exhaust system 70. The particle filter 204 serves forfiltering particles out of the exhaust-gas flow 203 in order to reduceemissions. The exhaust-gas flow 203 runs in the particle filter 204along an exhaust-gas path 203, which is provided with the same referencenumeral as the exhaust-gas flow 203.

For the filtering, the particle filter 204 comprises a filter 205, whichis designed here as a wall-flow filter. Arranged upstream of the filter205 is an inlet region 206 of the particle filter 204, which inletregion widens the cross section of the exhaust line 202 to the largercross section of the filter 205. Arranged downstream of the filter 205is an outlet region 207 which narrows the cross section again to thecross section of the exhaust line 202. The filter 205 and the particlefilter 204 are, overall, of rotationally symmetrical design with respectto an axis of rotation 208. The inner region of the filter 205, inparticular flow ducts and/or walls, need not be rotationallysymmetrical.

The filter 205 is surrounded by a secondary filter 209 which, like the(primary) filter 205, contains filter elements 210, for example ducts,walls and/or suitable material. The secondary filter 209 is structurallyseparate from the filter 205, that is to say there is no communicationof the exhaust flow 203 between the two filters 205 and 209. In oneexample, the secondary filter is adjacent to the primary filter andforms an annulus that surrounds the primary filter. The secondary filtermay have a longer length than the primary filter to account for thevariation in flow through its annulus as compared to through the oval orcircular cross-section of the primary filter. In addition, the cellsize, shape, and/or spacing of the secondary filter may differ from thatof the primary filter. In one example, the cell area of the secondaryfilter may be greater than that of the primary filter. Additionally, asingle can may form a housing around both the primary and secondaryfilter so only that a single inlet and outlet may be provided via thecan for exhaust flow.

One or more inlet valves 211 are arranged in the inlet region 206, whichinlet valve(s) connect the inlet region 206 and therefore the exhaustline 202 to the secondary filter 209. The inlet valves 211 may bedistributed over an inner circumference of the secondary filter 209. Oneor more outlet valves 212 are arranged in the outlet region 207, whichoutlet valves connect the secondary filter 209 to the outlet region 207and therefore to the exhaust line 202. The outlet valves 212 may bedesigned and/or arranged analogously to the inlet valves 211. Further,the exhaust flow may be in a direction to assist in opening the valvesinto and out of the secondary filter.

A temperature sensor 213 is arranged in the secondary filter 209 inorder to detect the temperature in the secondary filter 209. Thetemperature sensor 213 may also be arranged in the region of thesecondary filter 209, for example on an outer wall of the secondaryfilter 209 or on the outlet valve 212. Also arranged in the secondaryfilter 209 is a pressure sensor 214 by means of which the loading of thesecondary filter 209 can be detected on the basis of rising pressure.The pressure sensor 214, too, need not be arranged directly in thesecondary filter 209 but rather may be arranged for example on the inletvalve 211.

One or more sensors 215, of which two sensors are illustrated by way ofexample, detect the position of the inlet valve 211 and/or of the outletvalve 212. Depending on the number of inlet valves 211 and outlet valves212, a plurality of sensors 215 may be provided. It is also possible forone or more sensors 215 to detect only the position of the one or moreinlet valves 211 or only the position of the one or more outlet valves212. The particle filter 204 is illustrated with the sensors 213, 214and 215, but the particle filter 204 may self-evidently be formedentirely without sensors or may be equipped with only some of saidsensors.

The operation of the particle filter 204 will be described below on thebasis of FIGS. 2 and 3. When the filter 205 is fully functional, that isto say is not degraded, overloaded with particles, or where less than athreshold amount of particulate matter is stored in the primaryparticulate filter 204, the particle filter 204 is in a normal mode(e.g., FIG. 2). In said normal mode, the exhaust-gas counterpressuregenerated by the particle filter 204 is in a normal range which does notdegrade the engine or the exhaust system or cause a threshold reductionin engine power. The exhaust-gas flow 203 which passes through theexhaust line 202 flows through the filter 205 in order to be purified.During normal operation, therefore, the exhaust-gas path 203 runsthrough the filter 205.

During ongoing operation of the internal combustion engine, an everincreasing number of particles are accumulated in the filter 205, suchthat the loading of the filter 205 with particles continuouslyincreases, as a result of which the flow resistance of the filter 205increases. Consequently, the exhaust-gas counterpressure increases untilit reaches a value at which the performance of the internal combustionengine reduced more than a threshold amount or there is a risk ofdegradation to the internal combustion engine or the exhaust system. Theparticle filter 204 now switches into a secondary mode illustrated inFIG. 202, in which the exhaust-gas path 203, that is to say the path ofthe exhaust-gas flow 203, runs through the secondary filter 209.

A trigger or stimulus for the change of the operating mode may be theexhaust-gas counterpressure of the filter 205 and therefore of theparticle filter 204. The exhaust-gas counterpressure may be determinedeither by means of a pressure sensor 240 for example in the inlet region206 or in the exhaust line 202. If the exhaust-gas counterpressureexceeds a certain threshold, which may be predefined and/or variable,the inlet valves 211 open, such that the exhaust-gas path runs throughthe secondary filter 209. The inlet valves 211 may be either externallyactuated via actuator 250, for example by a motor or a mechanicalsystem, or internally opened and closed by means of a mechanism oractuator 250, such as a spring, arranged on or in the valve. In theexample with the spring, the sensor for the exhaust-gas counterpressuremay be omitted, because the valve opens automatically above anexhaust-gas counterpressure set by means of the spring force.

The exhaust-gas flow 203 is then filtered by the secondary filter 209.Similarly to the inlet valves 211, the outlet valves 212 open underincreasing pressure. If the pressure in the secondary filter 209 exceedsthe pressure in the outlet region 207, the spring-preloaded outletvalves 212 open, such that the exhaust-gas flow 203 circulates past thefilter 205. Here, it is not ruled out that a small part of theexhaust-gas flow 203 continues to circulate through the filter 205.While the exhaust-gas flow 203 runs through the secondary filter 209, aregeneration of the primary filter 205 can be carried out in order torestore the performance thereof.

Here, the secondary filter 209 is constructed such that, in the event ofan overloading of the secondary filter 209, the filtering efficiencythereof falls and the exhaust-gas counterpressure remains at a normallevel. In other words, this means that, even in the event of arelatively long period of operation of the secondary filter 209, theexhaust-gas counterpressure is not increased to such an extent that theperformance of the internal combustion engine is reduced or the internalcombustion engine is damaged.

Furthermore, the pressure sensor 214 may be used to detect the loadingof the secondary filter 209. In the event of an overloading, orimpending overloading, of the secondary filter 209, an activeregeneration of the secondary filter 209 may be initiated. Furthermore,an overloading mode may be indicated, and a regeneration or a filterexchange initiated. What measures are initiated may be made dependent onthe required opening pressure for the inlet valves 211 and/or the outletvalves 212. The level of filter loading may be derived from this.

Similarly, the activation of the secondary mode may be detected, and acorresponding reaction initiated, by means of the sensors 215 fordetecting the position of the inlet valve 211 and/or of the outlet valve212. The secondary mode may also be detected by means of the temperaturesensor 213. For this purpose, the temperature signal of the temperaturesensor 213, for example the dynamic profile of the signal, is comparedwith the temperature of the exhaust gas at the inlet of the exhaustaftertreatment system 70.

When the filter 205 has been regenerated and/or exchanged, whereindepending on the design of the particle filter 204, the filter 205 orthe complete particle filter 204 is exchanged, the exhaust-gas flow runsthrough the filter 205 again. Since the exhaust-gas counterpressure isat a normal level again when the filter 205 is in a correct state, theinlet valves 211 for the secondary filter 209 are closed. The sensorsand the status signals correspondingly indicate normal operation of theparticle filter 204 again. The sensors are connected to one or morecontrol units, which are either assigned exclusively to the particlefilter 204 or which contain information regarding the state of theparticle filter 204, such as for example control units for theexhaust-gas aftertreatment and/or for the engine management.

Referring now to FIG. 4, a method for operating an exhaust system thatcan direct engine exhaust to first and second particulate filters isshown. The method of FIG. 4 may be stored as executable instructions innon-transitory media such as memory in a system as is shown in FIG. 1.Method 400 may be executed when an engine is combusting an air-fuelmixture.

At 402, method 400 determines operating conditions. Operating conditionsmay include but are not limited to pressure upstream of the particulatefilter, pressure downstream of the particulate filter, engine speed,engine load, and particulate filter temperature. Method 400 proceeds to404 after operating conditions are determined.

At 404, method 400 directs substantially all engine exhaust to a primaryparticulate filter. In one example, the primary particulate filter is ofthe design shown in FIGS. 2 and 3. Thus, the primary particulate filteris the sole particulate filter receiving exhaust gases from the engine.Method 400 proceeds from 404 to 406.

At 406, method 400 determines an amount of particulate matter storedwithin the primary particulate filter. In one example, the amount ofparticulate matter is based on an observed pressure upstream of theparticulate filter. Specifically, empirically determined soot storageamounts are stored in a table in memory that is indexed via enginespeed, engine load or engine air flow, and exhaust pressure upstream.The table outputs an amount of soot stored in the particulate filer oralternatively a percentage of used soot storage capacity of theparticulate filter. Method 400 proceeds to 408 after the amount of sootstored in the particulate filter is determined.

At 408, method 400 judges whether or not the amount of particulatematter store in the particulate filter is greater than a thresholdamount. In one example, the amount of soot from 406 is compared to apredetermined soot amount. If the amount of soot stored in theparticulate filter is greater than the threshold amount, method 400proceeds to 410. Otherwise, method 400 proceeds to exit.

At 410, method 400 directs exhaust gas to the secondary particulatefilter. The exhaust gases may be directed via changing a position of avalve. In one example, the valve position is adjusted by overcoming aforce of a spring. In other examples, the valve may be opened via asolenoid or a motor. Method 400 proceeds to 412 after exhaust gases aredirected to the secondary particulate filter.

At 412, regeneration of the primary particulate filter may begin.Alternatively, an operator of the vehicle may be provided an indicationthat the primary particulate filter should be replaced. The primaryparticulate filter may be regenerated by increasing the temperature ofthe primary filter. In one example, the temperature of the primaryparticulate filter may be increased by increasing a temperature ofexhaust gases. The exhaust gas temperature may be increased viathrottling the engine and retarding fuel injection timing.Alternatively, if the engine is a spark ignited engine, spark timing maybe retarded to increase exhaust gas temperature. Further, the amount ofparticulate matter stored in the primary particulate filter may bedetermined as described at 406. In some examples, the valve that directsexhaust gases to the secondary particulate filter may be periodicallyclosed so that substantially no engine exhaust gases are directed to thesecondary particulate filter while the amount of soot stored in theprimary particulate filter is determined. Additionally, the inlet valvemay be periodically closed when the temperature of the first or secondparticulate filter exceeds a threshold temperature. Method 400 proceedsto 414 after regeneration or notification is provided.

At 414, method 400 judges whether or not particulate matter stored inthe primary particulate filter is less than a threshold amount. Theamount of soot determined at 412 is compared to a soot threshold amount.If the amount of soot determined at 412 is less than the soot thresholdamount, method 400 proceeds to exit. Otherwise, method 400 returns to410.

Thus, the method of FIG. 4 provides for an exhaust purification method,comprising: directing exhaust gases from an engine substantially solelyto a first particulate filter (PF) when a stored soot amount in thefirst PF is less than a threshold; and directing exhaust gases from theengine to a second PF when the stored soot amount in the first PF isgreater than the threshold, the second PF adjacent to and surroundingthe first PF. In this way, exhaust gases may be filtered even after anamount of soot stored in one particulate filter exceeds a thresholdamount.

The method further comprises combusting an air-fuel mixture in an engineto produce the exhaust gases, and where the amount of soot stored in thefirst particulate filter is determined from an increased exhaust-gascounterpressure. The method includes where during a condition where theamount of soot stored in the first particulate filter is greater than athreshold amount, a filtering efficiency of the first particulate filteris decreased from a nominal filtering efficiency and an exhaust-gascounterpressure remains below a threshold level. The method furthercomprises where the first particulate filter is regenerated when exhaustflow is directed to the second particulate filter, and furthercomprising opening and closing an inlet valve regulating flow to thesecond particulate filter during regeneration of the first particulatefilter. The method further comprises directing engine exhaust gas solelythrough the first particulate filter after the first particulate filteris regenerated.

In another example, the method of FIG. 4 provides for purification ofengine exhaust, comprising: combusting an air-fuel mixture in an engine;directing exhaust gases from the engine substantially solely to a firstparticulate filter when an pressure upstream of the first particulatefilter is less than a threshold amount; and directing exhaust gases fromthe engine to a second particulate filter when exhaust pressure upstreamof the first particulate filter overcomes a spring force, where thesecond particulate filter surrounds a portion of the first particulatefilter. The method also includes where exhaust gases are directed via aninlet valve and an outlet valve. The method further comprises where thefirst particulate filter is regenerated when exhaust gases are directedto the second particulate filter.

As will be appreciated by one of ordinary skill in the art, routinesdescribed in FIGS. 4 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

1. An exhaust system of a motor vehicle, comprising: a first particulatefilter; a second particulate filter; and a exhaust gas routing systemincluding a first exhaust passage through the first particulate filterand a second exhaust passage through the second particulate filter, aninlet valve biased in a closed position and located upstream of thefirst and second particulate filters.
 2. The exhaust system of claim 1,where exhaust gases are directed substantially solely to the firstparticulate filter when particulates stored in the first particulatefilter are less than a threshold level, and the exhaust gas routingsystem supplying exhaust gases to the second particulate filter whenparticulates stored in the first particulate filter are greater than thethreshold level.
 3. The exhaust system of claim 1, where the secondparticulate filter surrounds the first particulate filter.
 4. Theexhaust system of claim 1, further comprising a preload spring biasingthe inlet valve in a closed position.
 5. The exhaust system of claim 1,further comprising an outlet valve positioned downstream of the secondparticulate filter and the first particulate filter.
 6. The exhaustsystem of claim 5, further comprising a preload spring biasing theoutlet valve in a closed position.
 7. The exhaust system of claim 1,further comprising a pressure sensor positioned upstream of the firstparticulate filter.
 8. The exhaust system of claim 7, further comprisinga pressure sensor located downstream of the second particulate filter.9. The exhaust system of claim 1, further comprising an inlet valveposition sensor, an outlet valve position sensor, and a temperaturesensor positioned upstream of the second particulate filter.
 10. Anexhaust purification method, comprising: directing exhaust gases from anengine substantially solely to a first particulate filter (PF) when astored soot amount in the first PF is less than a threshold; anddirecting exhaust gases from the engine to a second PF when the storedsoot amount in the first PF is greater than the threshold, the second PFadjacent to and surrounding the first PF.
 11. The method of claim 10,further comprising combusting an air-fuel mixture in an engine toproduce the exhaust gases, and where the amount of soot stored in thefirst particulate filter is determined from an increased exhaust-gascounterpressure.
 12. The method of claim 11, where during a conditionwhere the amount of soot stored in the first particulate filter isgreater than a threshold amount, a filtering efficiency of the firstparticulate filter is decreased from a nominal filtering efficiency andan exhaust-gas counterpressure remains below a threshold level.
 13. Themethod of claim 10, further comprising where the first particulatefilter is regenerated when exhaust flow is directed to the secondparticulate filter, and further comprising opening and closing an inletvalve regulating flow to the second particulate filter duringregeneration of the first particulate filter.
 14. The method of claim13, further comprising directing engine exhaust gas solely through thefirst particulate filter after the first particulate filter isregenerated.
 15. A method for purification of engine exhaust,comprising: combusting an air-fuel mixture in an engine; directingexhaust gases from the engine substantially solely to a firstparticulate filter when an pressure upstream of the first particulatefilter is less than a threshold amount; and directing exhaust gases fromthe engine to a second particulate filter when exhaust pressure upstreamof the first particulate filter overcomes a spring force, where thesecond particulate filter surrounds a portion of the first particulatefilter.
 16. The method of claim 15, where exhaust gases are directed viaan inlet valve and an outlet valve.
 17. The method of claim 16, furthercomprising where the first particulate filter is regenerated whenexhaust gases are directed to the second particulate filter.